65 lines
1.6 KiB
C
65 lines
1.6 KiB
C
#include <math.h>
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#include <stdio.h>
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#include <stdlib.h>
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void swp(int i, int j, double xs[]){
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double tmp = xs[i];
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xs[i] = xs[j];
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xs[j] = tmp;
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}
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void array_print(double xs[], int n){
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printf("[");
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for(int i=0; i<n;i++){
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printf("%f, ", xs[i]);
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}
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printf("]\n");
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}
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int partition(int low, int high, double xs[], int length){
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// To understand this function:
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// - see the note after gt variable definition
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// - go to commit 578bfa27 and the scratchpad/ folder in it, which has printfs sprinkled throughout
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int pivot = low + floor((high-low)/2);
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double pivot_value = xs[pivot];
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swp(pivot, high, xs);
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int gt = low; /* This pointer will iterate until finding an element which is greater than the pivot. Then it will move elements that are smaller before it--more specifically, it will move elements to its position and then increment. As a result all elements between gt and i will be greater than the pivot. */
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for(int i=low; i<high; i++){
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if(xs[i] < pivot_value){
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swp(gt, i, xs);
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gt++;
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}
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}
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swp(high, gt, xs);
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return gt;
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}
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double quickselect(int k, double xs[], int length){
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int low = 0;
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int high = length - 1;
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for (;;){
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if(low == high){
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return xs[low];
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}
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int pivot = partition(low, high, xs, length);
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if(pivot == k){
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return xs[pivot];
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}else if(k < pivot){
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high = pivot - 1;
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} else {
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low = pivot + 1;
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}
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}
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}
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int main(){
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double xs[] = {2.1, 1.0, 6.0, 4.0, 7.0, -1.0, 2.0, 10.0};
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int length = 8;
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int k = 2;
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array_print(xs, 8);
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double result = quickselect(k, xs, length);
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printf("The item in pos #%d is: %f\n", k, result);
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array_print(xs, 8);
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return 0;
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}
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